From Norwegian patent NO 314296 owned by the present applicant, there is known a device for removing rust and paint on ships' hulls using induction heating. A portable induction heater unit is placed on the hull plate. Said unit includes an induction coil driven by a powerful signal generator. The magnetic field from the induction coil will set up eddy currents in the steel plate, which will be transformed to heat by the ohmic losses in the steel. The heat will lift the paint layers and rust due both to the temperature and differences in expansion coefficients. The supplied heat should be sufficient to lift the paint. However, overheating must be avoided to prevent scorching of the paint and the emission of unpleasant and unhealthy gases. Overheating may also be harmful for objects oil the inside of the plates, in particular if there are any inflammable gases present, and may even anneal the steel and change its properties in a undesirable way. Thus, it is very important to accurately control the supplied heat. The unit disclosed in NO 314296 is moved manually over the hull, and will naturally be moved with an uneven speed. To control the supplied heat, a tachometer wheel is mounted on the unit. The wheel traces the movement and controls the induction field, i.e. the unit is adapted to supply a controlled amount of energy per area. While the prior art system will control the supplied heat in a proper way under ideal conditions, it has a couple of shortcomings. Initially, the system must be manually set to the conditions prevailing on the ship in question, i.e. a mean value must be set that is adapted to the mean thickness of the paint layer. As the workers move to another part of the ship, these conditions may change due to changes in the thickness of the rust and paint, the thickness and the conductivity of the steel.